Add missing length check when copying fixed arrays.

This was mistakenly removed in r21027.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/259203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21037 ce2b1a6d-e550-0410-aec6-3dcde31c8c00